Electrical resistance heaters made from flexible electrically conductive fabrics are known in the art. Such heaters include heated blankets, heated clothing, heated car seats and the like. The electrically conductive elements in such heaters are made from carbon fibers, thin metal wires or foil. The carbon fibers or metal wires are formed into bundles and then woven into a mesh-like cloth or simply anchored to a backing material by sewing and/or sealing (e.g., by a potting material such as rubber). Electrical connections (e.g., via a bus bar) are usually coupled to opposite ends of the conductive cloth or conductive bundle.
The carbon fiber bundles and/or metal wires forming heater elements of this construction are typically flexible. Embodiments including carbon fiber bundles, for instance, retain mechanical properties at very high temperature (e.g., 1000 degree Celsius). However, while masses of carbon fibers potted in resin may be one of the strongest materials known, individual carbon fibers that are not potted in resin are easily fractured when they are flexed. Various manufacturers have developed a variety of ways of attempting to make carbon fibers more tolerant of flexing but the result is still a bundle of relatively fragile fibers that eventually fracture with repeated flexion and mechanical stress. The point of repeated flexion and stress is usually where the fabric heater is folded or bent. For instance, in the case of a heater for a car seat, the areas of the seat that maximally deflect are those that are subject to repetitive placement of a weight (e.g., a user's body).
When fiber bundle, resistance wires or foil heater elements develop repetitive mechanical stress, it may result in arcing due to a local region of higher resistance. For instance, if individual carbon fibers fracture in a given area, the electrical resistance increases in that area. This increase in resistance may either be gradual over time, or develop rapidly in the case of an acute fracture. As electrical currents preferentially follow the path of least resistance, the current may bypass the fractured area of higher resistance and flow via adjacent bundles. Thus, the area adjacent the fractured area develops higher current flow as the fracture evolves. This excess electrical current flow may result in excess heat being produced adjacent the fractured heater bundles, thereby creating localized areas of higher temperature (e.g., a hotspot) adjacent the fractured area.
As more and more fibers fracture, more and more electrical current is routed through the remaining conducting bundles of fibers. These bundles can become hot enough to burn (e.g., through a car seat) and sometimes cause discomfort or injury to a user (e.g., the occupant of the car seat). Such embodiments may develop hotspots in regions of maximum flexion or mechanical stress, which typically correlates with the point of contact between the user and the fabric heater (e.g., the point of contact between the occupant and the car seat). Thus, hotspots may develop in those areas where it is likely to contact the user and cause burn injuries.
In order to mitigate hotspot development in flexible heaters, fabric heater elements may be protected by providing a laminate. The laminate may include two or more layers of plastic film, woven or non-woven fabric, potting material such as silicone or a combination of these materials. The laminate may add mechanical strength to fabric heater to prevent fracture of fabric heater elements, and may provide both electrical insulation (especially via potting materials) and moisture protection to the heating element. However, the resulting heater may lose flexibility and elasticity, thereby preventing the fabric heater from being formed or disposed into complex shapes (e.g., a compound curve bending in two directions simultaneously). Such a heater construction with reduced flexibility and minimal elasticity may limit the usefulness of these heaters in applications such as warming blankets, warming mattresses and warmed clothing.